One of the more significant difficulties in learning a second language is the inability to recognize and identify some sounds, distinguishing the sounds from one another, when the distinction is not a component of a listener's native language. One example is the inability of a number of people who are native speakers of certain non-English languages, e.g., Japanese, to distinguish the sounds of the English language pertaining to the letters "r" and "l." Such people have great difficulty in distinguishing such words as (i) "rock" and "lock," (ii) "rake" and "lake," and (iii) "right" and "light." Other distinctions of the English language which are difficult for some non-native English speakers include distinctions between "b" and "v," between "f" and "t," and between "f" and "p." Such non-native English speakers would have difficulty distinguishing, or be unable to distinguish, "bat" from "vat" or "theses" or "pieces" from "feces." Such inability makes comprehension of a second language particularly difficult and hinders such a person's ability to speak in the second language. The result is often a heavy accent which is difficult for native speakers of the second language to comprehend.
Such difficulty in perceiving similar but distinct sounds of spoken language are not limited to people attempting to learn a second language. Some people have specific impairments, such as language-learning impairments, in which some distinctions between similar sounds are not perceptible.
What is needed is a mechanism by which a listener can be trained to perceive subtle differences in similar sounds which sound identical to the listener.